Vacuum IG units are known in the art. For example, see U.S. Pat. Nos. 5,664,395, 5,657,607, and 5,902,652, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1-2 illustrate a conventional vacuum IG unit (vacuum IG unit or VIG unit). Vacuum IG unit 1 includes two spaced apart glass substrates 2 and 3, which enclose an evacuated or low pressure space 6 therebetween. Glass sheets/substrates 2 and 3 are interconnected by peripheral or edge seal of fused solder glass 4 and an array of support pillars or spacers 5.
Pump out tube 8 is hermetically sealed by solder glass 9 to an aperture or hole 10 which passes from an interior surface of glass sheet 2 to the bottom of recess 11 in the exterior face of sheet 2. A vacuum is attached to pump out tube 8 so that the interior cavity between substrates 2 and 3 can be evacuated to create a low pressure area or space 6. After evacuation, tube 8 is melted to seal the vacuum. Recess II retains sealed tube 8. Optionally, a chemical getter 12 may be included within recess 13.
Conventional vacuum IG units, with their fused solder glass peripheral seals 4, have been manufactured as follows. Glass frit in a solution (ultimately to form solder glass edge seal 4) is initially deposited around the periphery of substrate 2. The other substrate 3 is brought down over top of substrate 2 so as to sandwich spacers 5 and the glass frit/solution therebetween. The entire assembly including sheets 2, 3, the spacers, and the seal material is then heated (typically using a convection oven) to a temperature of approximately 500° C., at which point the glass frit melts, wets the surfaces of the glass sheets 2, 3, and ultimately forms hermetic peripheral or edge seal 4. This approximately 500° C. temperature is maintained for from about one to eight hours. After formation of the peripheral/edge seal 4 and the seal around tube 8, the assembly is cooled to room temperature. It is noted that column 2 of U.S. Pat. No. 5,664,395 states that a conventional vacuum IG processing temperature is approximately 500° C. for one hour. Inventors Lenzen, Turner and Collins of the '395 patent have stated that “the edge seal process is currently quite slow: typically the temperature of the sample is increased at 200° C. per hour, and held for one hour at a constant value ranging from 430° C. and 530° C. depending on the solder glass composition.” After formation of edge seal 4, a vacuum is drawn via the tube to form low pressure space 6.
Unfortunately, the aforesaid high temperatures and long heating times of the entire assembly utilized in the formulation of edge seal 4 are undesirable, especially when it is desired to use a heat strengthened or tempered glass substrate(s) 2, 3 in the vacuum IG unit. As shown in FIGS. 3-4, tempered glass loses temper strength upon exposure to high temperatures as a function of heating time. Moreover, such high processing temperatures may adversely affect certain low-E coating(s) that may be applied to one or both of the glass substrates in certain instances.
FIG. 3 is a graph illustrating how fully thermally tempered plate glass loses original temper upon exposure to different temperatures for different periods of time, where the original center tension stress is 3,200 MU per inch. The x-axis in FIG. 3 is exponentially representative of time in hours (from 1 to 1,000 hours), while the y-axis is indicative of the percentage of original temper strength remaining after heat exposure. FIG. 4 is a graph similar to FIG. 3, except that the x-axis in FIG. 4 extends from zero to one hour exponentially.
Seven different curves are illustrated in FIG. 3, each indicative of a different temperature exposure in degrees Fahrenheit (° F.). The different curves/lines are 400° F. (across the top of the FIG. 3 graph), 500° F., 600° F., 700° F., 800° F., 900° F., and 950° F. (the bottom curve of the FIG. 3 graph). A temperature of 900° F. is equivalent to approximately 482° C., which is within the range utilized for forming the aforesaid conventional solder glass peripheral seal 4 in FIGS. 1-2. Thus, attention is drawn to the 900° F. curve in FIG. 3, labeled by reference number 18. As shown, only 20% of the original temper strength remains after one hour at this temperature (900° F. or 482° C.). Such a significant loss (i.e., 80% loss) of temper strength is of course undesirable.
In FIGS. 3-4, it is noted that much better temper strength remains in a thermally tempered sheet when it is heated to a temperature of 800° F. (about 428° C.) for one hour as opposed to 900° F. for one hour. Such a glass sheet retains about 70% of its original temper strength after one hour at 800° F., which is significantly better than the less than 20% when at 900° F. for the same period of time.
Another advantage associated with not heating up the entire unit for too long is that lower temperature pillar materials may then be used. This may or may not be desirable in some instances.
Even when non-tempered glass substrates are used, the high temperatures applied to the entire VIG assembly may soften the glass or introduce stresses, and partial heating may introduce more stress. These stresses may increase the likelihood of deformation of the glass and/or breakage.
Moreover, the ceramic or solder glass edge seals of conventional VIG units tend to be brittle and prone to cracking and/or breakage, reducing the ability of individual glass panels to move relative to one another. Glass panel movement is known to occur under normal conditions such as, for example, when two hermetically sealed glass components (such as in a VIG unit) are installed as a component of a window, skylight or door, whereby the VIG unit is exposed to direct sunlight and one glass panel has higher thermal absorption than the other panel or there is a great difference between the interior and exterior temperatures.
Currently, most frit slurry mixtures used in vacuum insulated glass (VIG) or plasma display panel (PDP) applications include organic solvents and binders, along with lead-borne ceramic powders that may be harmful to the environment and toxic to humans and/or other organisms. The base frits often contain large amounts of lead, most typically in the form of PbO. Additionally, most current frit slurry mixtures include hydrocarbon solvents, which present a number of environmental issues. The desire for a “lead-free” frit also is increasing, as some relevant rules and regulations are directed to the reduction and sometimes even complete banning of lead-based materials, e.g., in window applications.
Thus, it will be appreciated that there is a need in the art for a frit slurry mixture that is non-toxic and still capable of performing the desired or necessary functions of the potentially harmful slurry mixtures, and/or assemblies including the same.
In this regard, it will be appreciated that the use of water as a solvent reduces environmental concerns because, for example, water-based vehicles reduce production complexity and costs related to organic vapor burn-off and/or entrapment, recovery, recycling, and associated loss risks and the like. Certain example embodiments of this invention relate to the use of “lead-free” frit slurry mixtures that include FDA approved binders and water as a solvent, thereby reducing the toxic or environmentally hazardous nature of the slurry mixture in the wet and/or dried state(s). Certain example frit slurry mixtures are capable of being pumped, extruded, or otherwise disposed using conventional equipment, either manually, semi-automatically, or automatically. In certain example embodiments, the extruded material may essentially maintain its shape as it is extruded. The as-fired product in certain example embodiments may exhibit a reduced number of bubbles or stress anomalies, while also providing adequate adhesion to the substrate. Another example aspect of certain example embodiments relates to the ability of the frit/vehicle to remain stable, as premixed pastes may be desirable in certain example applications.
Certain example embodiments of this invention relate to a frit slurry paste comprising (1) a water-based solvent; (2) a bismuth- or ceramic-based frit powder, with the frit powder being substantially free from lead; and (3) a binder comprising methylcellulose at a concentration of 0.25%-5% by weight. The frit slurry paste has a bulk viscosity of 2,000-200,000 cps.
Certain example embodiments of this invention relate to an assembly comprising at least one substrate. A frit is formed by firing a frit slurry paste applied to the at least one substrate. The frit slurry paste comprises (1) a water-based solvent; (2) a bismuth- or ceramic-based frit powder, with the frit powder being substantially free from lead; and (3) a binder comprising a gelatinous material at a concentration of 0.001%-20% by weight. The frit slurry paste has a bulk viscosity of 2,000-200,000 cps.
Certain example embodiments of this invention relate to a method of making a frit slurry paste. Frit powder, binder material, and a water-based solvent are mixed together to form an intermediate mixture. The frit powder is substantially lead free, and the water-based solvent is provided at a first temperature. Additional water-based solvent may be added to the intermediate mixture to form a frit slurry paste. The additional water-based solvent may be provided at a second temperature, with the second temperature being lower than the first temperature, e.g., to obtain the desired binder concentration (which value may be given by weight percentage in certain example instances). The binder material is provided at a concentration of 0.001%-30% by weight with respect to the frit slurry paste or the frit slurry paste absent the frit powder. The frit slurry paste has a bulk viscosity of 2,000-200,000 cps.
Certain example embodiments of this invention relate to a method of making a vacuum insulated glass (VIG) unit. A first substrate (e.g., of glass) is provided. A frit slurry paste is applied around edges of the first substrate. A second substrate (e.g., of glass) is provided such that the first and second substrates are substantially parallel and spaced apart from one another and such that the frit slurry paste is in contact with the edges of the second substrate. The frit slurry paste is fired to create an edge seal. A cavity between the first and second substrates is at least partially evacuated. The frit slurry paste has a bulk viscosity of 2,000-200,000 cps and comprises: (1) a water-based solvent, (2) a bismuth- or ceramic-based frit powder, with the frit powder being substantially free from lead, and (3) a binder comprising methylcellulose at a concentration of 0.25%-5% by weight.
Certain example embodiments of this invention relate to a method of making a frit slurry paste. A binder material and hot water are mixed together so as to at least partially dissolve the binder material in the hot water, to form an intermediate mixture. Additional water is added to the intermediate mixture to form a vehicle, with the additional water being at a temperature below a temperature of the hot water. Frit powder is added to the vehicle in making the frit slurry paste. The binder material may be provided at a concentration of 0.001%-20% by weight with respect to the frit slurry paste or the frit slurry paste absent the frit powder, and the frit slurry paste may have a bulk viscosity of 2,000-200,000 cps. The adding of the frit powder to the vehicle may comprise slow stirring to reduce the likelihood of air entrapment in the frit slurry paste.
In certain example embodiments, this frit slurry paste may be used in a method of making an assembly comprising an edge seal formed on at least one substrate. The frit slurry paste may be applied to a surface of the at least one substrate. The frit slurry paste may be at least partially dried to at least partially remove the water and form a frit bead on the surface of the at least one substrate. The frit bead may be formed on the surface of the at least one substrate so as to burn out the binder material and sinter the resulting frit in making the edge seal of the assembly.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.